Transaction Authentication Number (TAN) is used by some online banking services as a form of single use One-Time passwords to authorize financial transactions.

Transaction Authentication Numbers provide additional security because they act as a form of Multi-Factor Authentication. Should the physical document or token containing the TANs be stolen, it will be of little use without the password; conversely, if the login data are obtained, no transactions can be performed without a valid Transaction Authentication Number.

Originally, Transaction Authentication Numbers were printed on a piece of paper and handed by the bank personnel to the end user. Now there are many different methods used for providing Transaction Authentication Numbers.